It is now common practice to use asynchronous electric motors and more specifically linear induction motors to move an object, in particular a vehicle travelling along a rail. However, the regulation of the speed of such a motor remains a problem, especially when speed must be maintained with great percision at a given level.
Generally, motor speed is controlled by regulating the frequency of the motor's supply.
For this purpose it has been proposed to use an inverter with an electronic device ensuring highly precise regulation of the frequency. However, the inverter and its related electronic devices are generally expensive and become prohibitive when they are intended to supply a high power asynchronous motor which is intended to drive heavy vehicles or is subjected to severe breaking, as is the case for example with submersible platforms used in a hydrodynamic test.
Systems have also been built in which the asychronous motor is powered by an alternator which is in turn is driven by a DC motor, thus permitting variations in the rotational speed of the alternator and thus of the asynchronous motor's power supply frequency. In this case, regulation is obtained by comparing the actual speed of the vehicle with a desired speed value, and by modifying the motor DC power supply as a function of the difference noted in the comparison. It is difficult however to obtain precise regulation of the speed of the asynchronous motor because of the inertia of cascaded electro-mechanical machines (D.C. motor-alternator).
Regulating speed by regulating the voltage of the power supply has also been tried. However, attempts of this type have been frustrated by the difficulty encounted in trying to rapidly vary power supply voltage, alternators being generally built to deliver steady voltage and having, for this reason, low internal reactance resulting from: the bulking character of magnetic circuits, the large space between the rotor and the stator, and the presence of devices for damping voltage fluctuations such as, for example, short circuits on rotor windings. Once again, rapid regulation of the speed of the motor and the object connected to it thus remains limited.